An antenna with physical dimensions less than the operating wavelength of the antenna, frequently called and “electrically small antenna” or “ESA”, exhibits complicated electrical impedances at the antenna feed-points. These complicated impedances prevent the efficient transfer of radio-frequency (RF) power to and from the antenna feed-points and the radiating electromagnetic fields and limit the throughput of data or information carried on the physical layer of wireless networks.
The following reference (REF) documents are incorporated by reference into the present disclosure as if fully set forth herein: i) REF1—M. Gustafsson, C. Sohl, and G. Kristensson, “Illustrations Of New Physical Bounds On Linearly Polarized Antennas”, IEEE Transactions on Antennas Propagation, vol. 57, pp. 1319-1327, May 2009; ii) REF2—R. E. Collins, “Foundations For Microwave Engineering”, Ch. 5, IEEE Press, 2001; iii) REF3—J. Holopainen, “Handheld DVB And Multisystem Radio Antennas”, Thesis, Helsinki University of Technology, April 2008; iv) REF4—X. Zhu and M. Brobston, “System And Method For A Digitally Tunable Impedance Matching Network”, U.S. Pat. No. 7,332,980, February 2008; v) REF5—M. Brobston, X. Zhu, and S. E. Kim, “System And Method For A Digitally Tunable Impedance Matching Network”, U.S. Pat. No. 7,671,693, March 2010; vi) REF6—G. Skahill, R. M. Rudish, J. A. Pierro, “Apparatus And Method For Broadband Matching Of Electrically Small Antennas”, U.S. Pat. No. 6,121,940, September 2000; vii) REF7—S. E. Sussman-Fort and R. M. Rudish, “Non-Foster Impedance Matching Of Electrically-Small Antennas”, IEEE Transactions on Antennas Propagation, vol. 57, pp. 2230-2241, August 2009; and viii) REF8—S. Y. Liao, Microwave Devices and Circuits, 3rd ed., Prentice-Hall, Inc., Englewood Cliffs, N.J., 1990.
As described in REF1, the feed-point impedances of electrically small antennas have several distinctive and undesirable characteristics: i) a small real part of the complex impedance (typically less than 50 ohms); ii) a negative imaginary part of the complex impedance (or capacitive impedance); iii) a narrow bandwidth; and iv) a large quality (or Q) factor.
Several technologies and techniques are currently used to address the feed-point impedance problems with ESA's. These include passive matching networks (e.g., REF2, REF3), tunable matching networks (TMN) (e.g., REF4, REF5), and negative impedance converters (NIC) circuits (e.g., REF6, REF7). All three forms involve some mechanism that transforms the undesirable impedance at the antenna feed-point to an impedance that conjugate matches that of the transceiver circuit connected to the antenna feed-point. Passive matching networks utilize fully passive components (inductors, capacitors, transformers, transmission lines, etc.) to achieve the impedance transformation but are limited to one or a few specific frequencies and may further reduce the operating bandwidth below that of the antenna alone.
Tunable matching networks (TMN) utilize variable components (e.g., variable capacitors and/or variable inductors) in conventional matching network topologies to provide frequency agile or flexible antenna matching that is not available in traditional passive matching networks. But, as with passive matching, the bandwidth of tunable matching networks may be less than the bandwidth of the antenna without matching and the bandwidth may be less than that specified by the application requirements. Negative impedance converter (NIC) circuits utilize active circuits that masquerade as negative passive components. Negative capacitors or inductors are placed in series, shunt, or combinations thereof with the feed-points of antennas to cancel out the imaginary part (capacitance or inductance) of the antenna impedances. Negative impedance circuit matching provides broadband matching at a single center frequency or within a single frequency band but cannot provide effective matching across multiple frequency bands.
However, new wireless and cellular equipment must operate across multiple and disparate frequency bands and, within those bands, must function across wide bandwidths. Present antenna matching technologies do not provide adequate or effective means to address both the bandwidth and frequency agility demanded of new wireless systems.